[mwtidd]

first you have to ask yourself what you want to be able to do,

do you want a drive system that is truly holonomic
(ie you need 4 motors)

or is merely strafing enough
(can be done with 2 motors and 2 fwd/rev transmissions)

or do you want a strafing and diagonal drive system.
(can be done with 2 motors, 2 fwd/rev transmissions, and 4 on/off transmission)